O connor et al 2015 1000 dams down & counting

496 1 MAY 2015 • VOL 348 ISSUE 6234 sciencemag.org SCIENCE

Precision medicine comes to psychiatry p. 499

Secure sustainable seafood from developing countries p. 504INSIGHTS

Goodbye to a large dam. Elwha River passing through the remains of Glines Canyon Dam on 21 February 2015. The

former Lake Mills can be seen in the background.

PERSPECTIVES

Forty years ago, the demolition of

large dams was mostly fiction, nota-

bly plotted in Edward Abbey’s novel

The Monkey Wrench Gang. Its 1975

publication roughly coincided with

the end of large-dam construction in

the United States. Since then, dams have

been taken down in increasing numbers

as they have filled with sediment, become

unsafe or inefficient, or otherwise outlived

their usefulness (1) (see the figure, panel

A). Last year’s removals of the 64-m-high

Glines Canyon Dam and the 32-m-high

Elwha Dam in northwestern Washington

State were among the largest yet, releasing

over 10 million cubic meters of stored sedi-

ment. Published studies conducted in con-

junction with about 100 U.S. dam removals

and at least 26 removals outside the United

States are now providing detailed insights

into how rivers respond (2, 3).

A major finding is that rivers are resil-

ient, with many responding quickly to

dam removal. Most river channels stabilize

within months or years, not decades (4),

particularly when dams are removed rap-

idly; phased or incremental removals typi-

cally have longer response times. The rapid

physical response is driven by the strong

upstream/downstream coupling intrinsic

to river systems. Reservoir erosion com-

monly begins at knickpoints, or short steep

By J. E. O’Connor, 1 J. J. Duda ,2

G. E. Grant3

Dam removals are reconnecting rivers in the United States

ECOLOGY

PHOTO: JOHN GUSSMAN/JGUSSMAN@DCPRODUCTIO

NS.COM

1000 dams down and counting

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1 MAY 2015 • VOL 348 ISSUE 6234 497SCIENCE sciencemag.org

reaches of channel, that migrate

upstream while cutting through

reservoir sediment. Substan-

tial fractions of stored reservoir

sediment—50% or more—can be

eroded within weeks or months

of breaching ( 4) (see the figure,

panel B). Sediment eroded from

reservoirs rapidly moves down-

stream ( 5, 6). Some sediment is

deposited downstream, but is of-

ten redistributed within months.

Many rivers soon trend toward

their pre-dam states ( 5, 7).

Migratory fish have also re-

sponded quickly to restored river

connectivity. Removal of a dam on

Virginia’s Rappahannock River in-

creased American eel populations

in Shenandoah National Park, 150

km upstream ( 8). Similarly, follow-

ing a small dam removal in Maine,

sea lamprey recolonized newly ac-

cessible habitat, increasing abun-

dance and nesting sites by a factor

of 4 ( 9). Within days of the blast

removing the last of Glines Can-

yon Dam, Elwha River Chinook

salmon swam upstream past its

rocky abutments. Responses have

been mixed for less mobile bot-

tom-dwelling plants and animals

in former reservoirs and down-

stream channels ( 10, 11).

Dam size, river size, reservoir

size and shape, and sediment

volume and grain size all exert

first-order controls on physical

and ecological responses to dam

removal. Larger dam removals

have had longer-lasting and more

widespread downstream effects

than the much more common small-dam

removals ( 4). Local environmental and habi-

tat conditions and the dam’s position in the

watershed also affect physical and ecologic

consequences. In the case of the Elwha

River, both dams were near the river mouth,

minimizing the extent of downstream ef-

fects while reconnecting large areas of high-

quality fish habitat upstream in Olympic

National Park.

Removals can also have additional con-

sequences, some of them unintended. For

example, changes to a headwater fish as-

semblage occurred when a removal allowed

upstream colonization by reservoir species

present behind a dam farther downstream

( 12). Watershed contaminants, organic accu-

mulations, nutrients, once-inundated struc-

tures, and landforms from past land uses

may be uncovered and sometimes mobilized

by dam removal.

Numerical and physical models have

guided removal and monitoring strategies,

forecast broad-scale trends, and helped

avoid negative outcomes ( 13), but cannot

yet predict fine-scale changes driving many

ecological processes. Quantitative models of

species and ecosystem responses to dam re-

moval lag even further behind.

Most dam-removal studies so far have

been short-duration and opportunistic. Most

dam-removal analyses are from the northern

United States. Few removals have markedly

altered flow and/or released large volumes of

fine sediment. Furthermore, studies truly in-

tegrating biological and physical responses

are rare. Common protocols, more coordi-

nation among disciplines, and longer, more

systematic monitoring and research would

benefit future syntheses ( 13).

Coming down. (A) U.S. dam removals by decade. Data from ( 1). (B)

Rates of reservoir sediment erosion for 16 recent U.S. dam removals.

Condit, Marmot, Glines Canyon, and Elwha dams impounded sand-

rich sediment accumulations and were removed over short periods

ranging from hours to 3 years, leading to rapid reservoir sediment

erosion. Stronach Dam was removed in several phases over 7 years,

slowing reservoir erosion ( 15). Data from ( 4).

10.1126/science.aaa9204

1U.S. Geological Survey, Portland, OR, USA. 2U.S. Geological Survey, Seattle, WA, USA. 3USDA Forest Service, Corvallis, OR, USA. E-mail: [email protected]

In the United States, many dam remov-

als have improved ecosystem function while

avoiding catastrophic consequences to either

ecosystems or human uses. The high pace

of dam removal will likely continue. But the

future is murky. As mostly small dams con-

tinue to come down, dam-removal advocates

will gaze up at the many large and ecologi-

cally disruptive dams across the country that

are decaying and filling with sediment. Deci-

sions regarding these dams will require bal-

ancing risks, continued economic function,

and the potential for ecologic restoration.

Also clouding the future is climate change,

which is likely to increase the demand for

fresh-water storage, both as a low-carbon en-

ergy source and for consumptive use.

Dams are also being removed internation-

ally; the 26 removals with published studies

are just a sample from a total probably num-

bering in the hundreds. Like most of those in

the United States, many are small structures

at the end of their useful lives. And many re-

movals, such as the ongoing one of Japan’s

Arase Dam, are motivated by economic and

ecological considerations similar to those

spurring U.S. dam removal.

The total number of U.S. and international

removals are, however, more than offset by a

renewed global boom in dam construction,

chiefly for hydropower and in regions with

emerging economies, such as Southeast Asia,

South America, and Africa ( 14). But the dams

of this ongoing boom will also age, just like

those of the U.S. dam-building heyday. Dam

removal looks like an activity with a long fu-

ture ahead. ■

REFERENCES AND NOTES

1. www.americanrivers.org/initiatives/dams/dam-removals-map

2. A. G. Lejon, B. M. Renöfält, C. Nilsson, Ecol. Soc. 14, 4 (2009). 3. J. R. Bellmore et al., USGS Dam Removal Science Database

(2015); http://doi.org/.10.5066/F7K935KT. 4. G. E. Grant, S. L. Lewis, in Engineering Geology for Society

and Territory, vol. 3, G. Lollino et al., Eds. (Springer, Switzerland, 2015), pp. 31–35.

5. J. J. Major et al., Geomorphic response of the Sandy River, Oregon, to removal of Marmot Dam: U.S. Geological Survey Professional Paper 1792 (2012).

6. A. J. Pearson, N. P. Snyder, M. J. Collins, Water Resourc. Res. 47, W08504 (2009).

7. A. E. East et al., Geomorphology 228, 765 (2015). 8. N. P. Hitt et al., Trans. Am. Fish. Soc. 141, 1171 (2012). 9. R. Hogg, S. Coghlan Jr., J. Zydlewski, Trans. Am. Fish. Soc.

142, 1381 (2013). 10. D. D. Tullos et al., PLOS ONE 9, e108091 (2014). 11. C. H. Orr et al., River Res. Appl. 24, 804 (2008). 12. M. S. Kornis et al., Aquat. Sci. 10.1007/s00027-014-0391-2

(2014). 13. P. W. Downs et al., Int. J. River Basin Manag. 7, 433 (2009). 14. C. Zarfl, A. E. Lumsdon, J. Berlekamp, L. Tydecks, K.

Tockner, Aquat. Sci. 77, 161 (2015). 15. B. A. Burroughs et al., Geomorphology 110, 96 (2009).

ACKNOWLEDGMENTS

This Perspective is derived from discussions and analysis efforts conducted by the working group on Dam removal: Synthesis of ecological and physical responses of the U.S. Geological Survey John Wesley Powell Center for Analysis and Synthesis.

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Dam removals in the United States

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